I think the A’s are ganging up on me or are scared I may take them out in a rain shower. The A7 developed a leaky petrol tap (main) on Saturday, so no spares available over bank holiday weekend. I read and read and search and searched and found a some advise to steam the old cork. Well did it, it swelled and sealed and still is. Don’t know how long it will last, so will buy a spare this week but a quick fix that allowed a nice ride during a sunny spell .
It is almost a routine job on the m20.
Boiling the petrol taps.
As for the rubber "corks"
I would be inclined to pop them in some "fuel" in a sealed glass container and leave them in a warm spot for a couple of days and see how they cope.
There is only 3 grades of rubber that are truely petrol proof.
There are a a lot more that are "petrol resistant", these will go soft after a few days in the sun.
AFAIK only teflon ( and its derivatives ) are both ethanol proof and petrol proof.